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Kennedy Jenks <br /> 13 April 2021 <br /> Technical Memorandum <br /> To: Kyle Bringedahl, The Boeing Company <br /> From: Ben Fuentes, PE <br /> Subject: Flow Control and Treatment Criteria Design Summary <br /> Boeing Everett Site Stormwater Upgrade <br /> KJ 2096009.00 <br /> 1.1 Summary <br /> The Boeing Everett Stormwater Retrofit project proposes changes to the existing treatment and <br /> detention basin system to meet the new numeric effluent discharge limits (NELs) for copper and <br /> zinc from the Washington Industrial Stormwater General Permit (ISGP) effective January 1, <br /> 2020. This technical memorandum summarizes the proposed design and demonstrates how it <br /> meets or exceeds the requirements of the existing system. <br /> Design requirements: <br /> 1. Flow restrictions: <br /> a. Discharges to Powder Mill Gulch (PMG) shall not exceed 49 cubic feet per second (cfs) <br /> in the 25-year event. <br /> b. Discharges to the wetland downstream of the treatment basin shall not exceed 8 cfs. <br /> 2. Stormwater treatment: The new treatment media must provide an equivalent or better level <br /> of treatment compared to the existing peat filter for the parameters of concern in the <br /> stormwater manual and the NELs. <br /> 3. Treatment Volume: The upgraded facility shall treat at least as much stormwater volume as <br /> the existing facility. <br /> 1.2 Existing Stormwater System <br /> The existing stormwater flow control and treatment system consists of a Sedimentation Basin, <br /> Detention Basin, and peat filter treating approximately 887 acres from the Boeing Everett facility <br /> (Facility). <br /> In the existing system, stormwater enters the sedimentation basin for initial settlement of <br /> suspended solids. Up to 8 cfs of this pretreated water discharges to the peat filter for treatment. <br /> The peat filter discharges 8 cfs to the Powder Mill Gulch wetlands, which discharge to Powder <br /> Mill Creek. The remainder of water that enters the sedimentation basin discharges over the <br /> 1kkjc.locallkjc-rootlkj-projectslfederalway12020t2096009.00 boeing everett sw treatment109-reports19.08-reports-tmslcity of everett design tftoeing everefl sw design tech memo.docx !Kennedy/Jenks Consultants. <br /> Inc. <br />